For certain applications, motors are an essential piece of electrical equipment, in large, industrial facilities and residential buildings alike. They are used in a wide range of applications—from the large three-phase induction motors that the drive reactor coolant pumps in nuclear generation stations, to the small universal motors that drive a vacuum cleaner. Motors are a crucial component of every nation's economy not only because of the work they perform, but also because of the considerable amount of energy they consume.
The most commonly used type of motor is a polyphase induction motor with over 90% of those being squirrel-cage induction motors. Polyphase induction motors are popular for several reasons including: they are relatively inexpensive; they enjoy a rudimentary design; they are readily replaced; they have reliable operation; and they have a range of mounting styles and environmental enclosures.
Due to the significant capital and operational investments made by enterprises in motors—investments that impact the bottom line—knowing the state of their condition is vital. Induction motors are generally robust, but they can fail prematurely. Causes of motor failures include poor maintenance practices, improper lubrication, harsh operating environment, inadequate source voltage, or misapplication of the motor. All of these issues have one commonality: excessive temperature rise. Excessive heat is the nemesis of motors; temperature rise can originate in the bearings (lubrication, alignment, etc.), in the windings (design, voltage, etc.), or can be imposed by external conditions (ambient temperature, atmosphere, etc.).
One way of monitoring the health of a motor is to monitor the current used by the motor. These monitoring techniques do not account for variations in the voltage that can affect the inrush current and the full-load current (FLA). The inrush (or locked-rotor) current is the current drawn by the motor when it is initially started up from a stopped position. The actual inrush current value is typically much higher than the rated full-load current, and is usually stated by the manufacturer of the motor on the nameplate as the locked-rotor current. Many operators correctly assume that as a motor's terminal voltage decreases below its rated voltage, the motor's inrush and full-load currents will increase. However more counter-intuitively, if the motor's terminal voltage increases above its rated voltage, the motor's inrush and full-load currents will also increase. Misunderstanding the relationships between voltages and currents can result in misdiagnosed motor conditions or assumptions that an induction motor is operating within normal range.
Known motor monitoring schemes do not account for the relationships of high and low voltage motor terminal variations with the motor's startup and run currents. They either assume that whatever variation in the voltage that exists contributes a negligible effect on the motor's performance or assume that the actual voltage across the motor's power terminals is constant relative to the rated voltage. In real world induction motors, its terminal voltage varies and can have a significant impact on motor performance that can indicate a potential mechanical problem with the motor. More importantly, not accounting for variations at the motor's terminals can provide misleading conclusions regarding the motor's health.